This invention relates to high-temperature machine components. More particularly, this invention relates to coating systems for protecting machine components from exposure to high-temperature environments. This invention also relates to methods for protecting articles.
High-temperature materials, such as, for example, ceramics, alloys, and intermetallics, offer attractive properties for use in structures designed for service at high temperatures in such applications as gas turbine engines, heat exchangers, and internal combustion engines, for example. However, the environments characteristic of these applications often contain reactive species, such as water vapor, which at high temperatures may cause significant degradation of the material structure. For example, water vapor has been shown to cause significant surface recession and mass loss in silicon-bearing materials. The water vapor reacts with the structural material at high temperatures to form volatile silicon-containing species, often resulting in unacceptably high recession rates.
Environmental barrier coatings (EBC's) are applied to silicon-bearing materials and other material susceptible to attack by reactive species, such as high temperature water vapor; EBC's provide protection by prohibiting contact between the environment and the surface of the material. EBC's applied to silicon-bearing materials, for example, are designed to be relatively stable chemically in high-temperature, water vapor-containing environments. One exemplary conventional EBC system, as described in U.S. Pat. No. 6,410,148, comprises a silicon or silica bond layer applied to a silicon-bearing substrate; an intermediate layer comprising mullite or a mullite-alkaline earth aluminosilicate mixture deposited over the bond layer; and a top layer comprising an alkaline earth aluminosilicate deposited over the intermediate layer. In another example, U.S. Pat. No. 6,296,941, the top layer is a yttrium silicate layer rather than an alumino silicate.
The above coating systems can provide suitable protection for articles in demanding environments, but opportunities for improvement in coating performance exist. Current EBC technology generally uses plasma spray processes to deposit the coatings, primarily because of the flexibility of the process to deposit a large variety of materials, its ability to provide a wide spectrum of coating thicknesses without major process modifications, and the relative ease of depositing a coating layer. However, ceramic coatings processed by plasma spraying often contain undesirable open porosity in the form of a network of fine cracks (“microcracks”) intercepting otherwise closed pores and voids. The microcrack network is formed primarily by quench and solidification cracks and voids inherent in the coating deposition process; cracks often form between layers of successively deposited material and between the individual “splats” formed when melted or partially melted particles are sprayed onto the coating surface. For EBC applications, open porosity in the coating can be detrimental. It provides a rapid path for penetration of water vapor and other gaseous species and, hence, accelerated localized deterioration of the underlying coating layers.
Various methods have been implemented to alleviate the problem of open porosity in ceramic coatings. In some applications, the coatings are applied onto a hot substrate (T>800 C) using plasma spray processing. Deposition on a hot substrate reduces the difference between the substrate temperature and the melting temperature of the coating material, and thus reduces the tendency for formation of quench cracks. However, extension of the hot deposition process technique to large components is challenging, owing to the high substrate temperatures and the constraints associated with manipulation of the parts and the coating hardware. In other applications, the plasma sprayed EBC coating is submitted to a post-deposition process to impregnate the non-hermetic coating structure with precursors of suitable materials, for example, soluble organic and inorganic salts and alcoxides that yield upon heat-treatment a final pore-filling material compatible with the coating matrix. The filler material blocks or restricts the pathway for water vapor penetration. Such a process is described in U.S. patent application Ser. No. 11/298,735. Although this method is relatively easy to implement, it may require multiple impregnation-burnout cycles to achieve coating permeability improvements, and in certain cases may provide an incompletely hermetic coating structure.
Therefore, there is a need for articles protected by robust coating systems having improved capability to serve as a barrier to water vapor and other detrimental environmental species. There is also a further need for methods to produce these articles economically and reproducibly.